Advisories

WDPN33 PGTW 292100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (BANG-LANG) WARNING NR 015//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 27.7N 169.2E
   INITIAL INTENSITY: 65 KTS
   GEOGRAPHIC REFERENCE: 524 NM NORTH-NORTHEAST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 24 KTS
   SIGNIFICANT WAVE HEIGHT: 25 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TYPHOON
(TY) 20W (BANG-LANG) WITH QUICKLY COLLAPSING DEEP CONVECTION AND
CLOUD TOPS WARMING TO ABOVE -60C WITHIN THE LAST FEW HOURS.
ADDITIONALLY, THE LOW-LEVEL CIRCULATION CENTER APPEARS TO BE BARELY
PEEKING OUT FROM UNDER THE NORTHWESTERN PERIPHERY OF THE CENTRAL
DENSE OVERCAST. THE 291706Z SSMIS MICROWAVE DATA REVEALS THE
STRUCTURE OF THE STORM HAS DEGRADED; BOTH THE UPPER AND LOWER
LEVELS ARE DISORGANIZED COMPARED TO SEVERAL HOURS AGO. THERE IS NOT
EVEN A HINT OF A MICROWAVE EYE IN THE 37 GHZ MICROWAVE IMAGE.
ENVIRONMENTAL ANALYSIS REVEALS A MARGINALLY UNFAVORABLE ENVIRONMENT
CHARACTERIZED BY HIGH (25-30 KTS) VERTICAL WIND SHEAR (VWS),
MARGINAL (0-20 KJ PER CM2) OCEAN HEAT CONTENT (OHC), AND WEAK DRY
AIR ENTRAINMENT, SLIGHTLY OFFSET BY DECENT EASTWARD OUTFLOW AND
WARM (27-28 C) SEA SURFACE TEMPERATURES (SST). THE INITIAL POSITION
IS PLACED WITH MEDIUM CONFIDENCE BASED ON A COMBINATION OF
LOW-LEVEL CLOUD BANDING IN ANIMATED EIR AND AGENCY POSITION FIXES.
THE INITIAL INTENSITY OF 65 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON THE SUDDEN AND INTENSE COLLAPSE IN DEEP CONVECTION, CIMSS
AUTOMATED INTENSITY ESTIMATES, AND AGENCY DVORAK FIXES.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WESTERN FLANK OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED NEAR THE DATELINE


AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   RJTD: T4.0 - 65 KTS
   RCTP: T4.0 - 65 KTS
   KNES: T4.5 - 77 KTS
   PHFO: T4.5 - 77 KTS
   CIMSS SATCON: 62 KTS AT 291700Z
   CIMSS ADT: 47 KTS AT 291900Z
   CIMSS AIDT: 43 KTS AT 291900Z
   CIMSS D-MINT: 47 KTS AT 291707Z
   CIMSS D-PRINT: 47 KTS AT 291900Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 25-30 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE EASTWARD
   OTHER FACTORS: WEAK MID-LEVEL DRY AIR ENTRAINMENT

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: LOW

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TY 20W WILL BEGIN TO TRACK NORTH-NORTHWESTWARD
IMMINENTLY, AS THE STEERING STR TO ITS EAST IS WEAKENED BY A
PASSING UPPER-LEVEL TROUGH AND 20W CONTINUES TO INTERACT WITH
TROPICAL STORM (TS) 21W TO ITS NORTHWEST. BETWEEN TAU 24-36, 20W IS
EXPECTED TO CURVE TO THE NORTHEAST AS IT BEGINS SUBTROPICAL
TRANSITION AND GETS PICKED UP BY THE UPPER-LEVEL TROUGH.
SUBTROPICAL TRANSITION IS FORECAST TO BEGIN AROUND TAU 36 AND
COMPLETE NO LATER THAN TAU 48. REGARDING INTENSITY, 20W IS EXPECTED
TO QUICKLY WEAKEN THROUGH TAU 36, AND SUSTAIN ITS INTENSITY FROM
TAU 36-48 AS A SUBTROPICAL SYSTEM. THE ENVIRONMENT WILL GET
INCREASINGLY HOSTILE AS 20W TRACKS POLEWARD, WITH VWS EXPECTED TO
INCREASE FROM 25 KTS TO 55 KTS OVER THE COURSE OF THE FORECAST
PERIOD. CONSEQUENTLY, DRY AIR IS EXPECTED TO FURTHER ENTRAIN INTO
THE CORE, ESPECIALLY AFTER TAU 24. 20W IS EXPECTED TO CROSS THE 26C
ISOTHERM AROUND TAU 12, HASTENING WEAKENING THEREAFTER. 

MODEL DISCUSSION: DETERMINISTIC MODEL TRACK GUIDANCE IS IN FAIR
AGREEMENT THROUGH THE FORECAST PERIOD, THOUGH THERE IS A DISTINCT
SPREAD BETWEEN HISTORICALLY RELIABLE MODELS -- GFS, ECMWF, AND
EXPERIMENTAL AI MODELS -- AND THE HISTORICALLY WEAKER PERFORMING
MODELS -- UKMET, GSM, NAVGEM, AND GALWEM -- THE LATTER GROUP
HEAVILY INFLUENCING THE MULTI-MODEL CONSENSUS. AS A RESULT, THE
JTWC TRACK FORECAST IS PLACED IN ALIGNMENT WITH THE GROUPING OF
RELIABLE MODELS WITH OVERALL MEDIUM CONFIDENCE. RELIABLE INTENSITY
AIDS ARE IN GOOD AGREEMENT IN A SWIFT WEAKENING TREND THROUGH TAU
36, WITH SLIGHT DISAGREEMENT IN INTENSITY ONCE 20W BEGINS
SUBTROPICAL TRANSITION AT TAU 36. AS A RESULT, THE JTWC INTENSITY
FORECAST IS ASSESSED WITH OVERALL MEDIUM CONFIDENCE.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM//
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